The invention relates to alkali halide scintillation crystals, and more particularly relates to methods by which alkali halide scintillation crystals may be manufactured. In its most immediate sense, the invention relates to hot forging of NaI(Tl) scintillation crystals so as to produce rectangular scintillators having a large area, a rectangular shape, and uniform properties.
U.S. Pat. No. 5,792,253 (hereinafter, the "'253 patent") discloses a method of forging a cylindrical ingot of an alkali halide. In accordance with that method, a melt-grown ingot is flatted parallel both to its axis and to a particular crystallographic plane; the particular plane is determined by the crystal structure of the alkali halide to be forged. Then, the flat is placed on the lower platen of a heated dual-platen press and the ingot is compressed (advantageously, by raising the lower platen) between the platens while in a plastic state. The method produces a forged ingot in which cracks and fissures are absent from the periphery of the ingot.
Although this method achieves its intended objective, under certain circumstances the method can produce ingots having inhomogeneous characteristics (such as transparency and scintillation efficiency). For example, when NaI is doped with a Tl activator, the Tl content in the melt may be unstable during the growth process. The same is true for the content of impurities. As a result, the melt-grown ingot will be correspondingly inhomogeneous, and the inhomogeneity will be most pronounced along the growth direction of the ingot (i.e. along the axis of the ingot as it emerges from the melt). Hence, when the ingot is forged, the inhomogeneity of its composition can cause the characteristics of the forged ingot to progressively change from one region of the forged ingot to another. This adversely affects the performance of scintillation crystals made from the forged ingot.
In accordance with the invention, the melt-grown alkali halide ingot is still heated to plasticity and compressed between the heated platens of a dual-platen press, but before the compression begins the ingot is placed mid-way between two parallel, planar, and vertically extending barriers. The ingot is positioned so that its axis is vertical and so that the barriers are maintained in a predetermined relationship with the crystal lattice structure of the ingot. (If the ingot is of NaI, which has a face-centered crystal lattice, the barriers are parallel to the (100) crystallographic plane or to the (010) crystallographic plane. If the ingot is of CsI, which has a body-centered crystal lattice, the barriers are parallel to the (100) crystallographic plane.) Then, the ingot is compressed along its axis to form a block. (If the ingot is of NaI, this axis will generally coincide with the [001] crystallographic direction. If the ingot is of CsI, this axis will generally coincide with the [011] crystallographic direction.)
Because the ingot is compressed along the direction in which its inhomogeneities have the most pronounced effect on the characteristics of the ingot, the block is more uniform along its length and width. The block is therefore more likely to produce scintillation crystals suitable for use in scintillation cameras.
Advantageously, the barriers are connected together by transversely extending frame elements to form a rectangular frame. Advantageously, the distance between the barriers is less than the distance between the frame elements, which causes the block to have a generally rectangular shape. Further advantageously, the frame is suspended from the upper platen by cables that are counterbalanced by weights. This keeps the frame aligned with the thickest portion of the ingot as the forging process proceeds.
Alternatively, if the ingot is supported on the lower platen and its upper end is substantially narrower than its lower end (as is the case when the growth process comprises gradual broadening of the ingot in the first stage or gradual narrowing of the ingot in the final stage) the frame can be placed on the lower platen and can rest upon the lower platen as compression proceeds. Ideally, the ingot as pulled from the melt is a single crystal, i.e. has an unvarying crystallographic structure. However, this is not absolutely necessary. Ingots made up of more than one crystal can also be forged. Such forging can be done as long as it is possible, in an overall sense, to orient the crystallographic structure of the ingot with respect to the axis of compression, i.e. as long as all the crystals are not severely misaligned with respect to each other.